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+/*
+ * nghttp2 - HTTP/2 C Library
+ *
+ * Copyright (c) 2012 Tatsuhiro Tsujikawa
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining
+ * a copy of this software and associated documentation files (the
+ * "Software"), to deal in the Software without restriction, including
+ * without limitation the rights to use, copy, modify, merge, publish,
+ * distribute, sublicense, and/or sell copies of the Software, and to
+ * permit persons to whom the Software is furnished to do so, subject to
+ * the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+ * LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+ * OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+#ifndef SHRPX_DOWNSTREAM_QUEUE_H
+#define SHRPX_DOWNSTREAM_QUEUE_H
+
+#include "shrpx.h"
+
+#include <cinttypes>
+#include <map>
+#include <set>
+#include <memory>
+
+#include "template.h"
+
+using namespace nghttp2;
+
+namespace shrpx {
+
+class Downstream;
+
+// Link entry in HostEntry.blocked and downstream because downstream
+// could be deleted in anytime and we'd like to find Downstream in
+// O(1).  Downstream has field to link back to this object.
+struct BlockedLink {
+  Downstream *downstream;
+  BlockedLink *dlnext, *dlprev;
+};
+
+class DownstreamQueue {
+public:
+  struct HostEntry {
+    HostEntry(ImmutableString &&key);
+
+    HostEntry(HostEntry &&) = default;
+    HostEntry &operator=(HostEntry &&) = default;
+
+    HostEntry(const HostEntry &) = delete;
+    HostEntry &operator=(const HostEntry &) = delete;
+
+    // Key that associates this object
+    ImmutableString key;
+    // Set of stream ID that blocked by conn_max_per_host_.
+    DList<BlockedLink> blocked;
+    // The number of connections currently made to this host.
+    size_t num_active;
+  };
+
+  using HostEntryMap = std::map<StringRef, HostEntry>;
+
+  // conn_max_per_host == 0 means no limit for downstream connection.
+  DownstreamQueue(size_t conn_max_per_host = 0, bool unified_host = true);
+  ~DownstreamQueue();
+  // Add |downstream| to this queue.  This is entry point for
+  // Downstream object.
+  void add_pending(std::unique_ptr<Downstream> downstream);
+  // Set |downstream| to failure state, which means that downstream
+  // failed to connect to backend.
+  void mark_failure(Downstream *downstream);
+  // Set |downstream| to active state, which means that downstream
+  // connection has started.
+  void mark_active(Downstream *downstream);
+  // Set |downstream| to blocked state, which means that download
+  // connection was blocked because conn_max_per_host_ limit.
+  void mark_blocked(Downstream *downstream);
+  // Returns true if we can make downstream connection to given
+  // |host|.
+  bool can_activate(const StringRef &host) const;
+  // Removes and frees |downstream| object.  If |downstream| is in
+  // DispatchState::ACTIVE, and |next_blocked| is true, this function
+  // may return Downstream object with the same target host in
+  // DispatchState::BLOCKED if its connection is now not blocked by
+  // conn_max_per_host_ limit.
+  Downstream *remove_and_get_blocked(Downstream *downstream,
+                                     bool next_blocked = true);
+  Downstream *get_downstreams() const;
+  HostEntry &find_host_entry(const StringRef &host);
+  StringRef make_host_key(const StringRef &host) const;
+  StringRef make_host_key(Downstream *downstream) const;
+
+private:
+  // Per target host structure to keep track of the number of
+  // connections to the same host.
+  HostEntryMap host_entries_;
+  DList<Downstream> downstreams_;
+  // Maximum number of concurrent connections to the same host.
+  size_t conn_max_per_host_;
+  // true if downstream host is treated as the same.  Used for reverse
+  // proxying.
+  bool unified_host_;
+};
+
+} // namespace shrpx
+
+#endif // SHRPX_DOWNSTREAM_QUEUE_H